Ion sources for mass spectrometers



Nov. 1, 1960 E. J. ROBBINS ETAL 2,

ION SOURCES FOR MASS SPECTROMETERS Filed Oct. 8, 1958 Inventors: Eric John Robbins and J am :1

Alax McKnight $01 37 W Att orneys ION SOURCES FOR MASS SPECTROMETERS Eric John Robbins, Bromborough, and James Adan McKnight, Chester, England, assignors to the United Kingdom Atomic Energy Authority, London, England Filed Oct. 8, 1958, Ser. No. 766,074

Claims priority, application Great Britain Oct. 11, 1957 3 Claims. (Cl. 250'41.9)

This invention relates to ion sources for mass spectrometers and it provides an ion source suitable for use with gaseous samples such as uranium hexafluoride.

In the analysis of the isotopic concentration of uranium hexafluoride using a mass spectrometer having a conventional ion source, two adverse effects are noticed. First, after a certain length of time, the measured ion beams becomes unstable due to the formation of insulating reaction products on the walls of the source. Second, an effect sometimes referred to as the memory eifect has been observed, that is, the presence of minute quantities of one sample affects readings obtained on a following sample. The effect is thought to be due to adherence of traces of the earlier sample on the walls of the ion source which are subjected to U -U isotopic exchange when the following sample is introduced. To reduce memory effect to an acceptable level it is necessary to carry out a prolonged decontamination operation of the ion source in the mass spectrometer, decontamination being regarded as reaching a satisfactory point when readings obtained from the spectrometer are stable. The decontamination operation seriously reduces the time available for the spectrometer to carry out analysis of samples.

According to the invention an ion source for a mass spectrometer comprises means providing an ionizing beam consisting of a beam of electrons, means providing molecular beam flow of material to be ionized across said ionizing beam, and an electrode of skeletal form providing a small surface area but maintaining a uniform electric field along the path of the ionizing beam where it traverses the molecular flow beam.

The skeletal form of the ion source of the invention with its consequent reduction of surface area and increase in the pumping speed of material from the ion source gives reduced corrosion. In this way the memory effect is reduced and the ion source life is increased. In order to compensate for the loss in sensitivity (electron beam size/unit gas flow) the ionizing beam of electrons is increased by a large factor (cg. ten times) over conventional practice and the incoming material to the ion source is beamed into the ionizing beam by passing it through a hole, the diameter of which is less than its length, i.e. a molecular beam is formed.

By Way of example, the invention will now be described with reference to the single figure of the accompanying drawing which is an isometric view.

In the drawing an ion source 1 comprises a cathode 2, an anode 3 and a plate 4 on which is mounted an elec- 2,958,775. Patented Nov. 1, 1960 2 trode 5. A hole 6 is provided through the plate 4 and the electrode 5.

The hole 6 has a diameter equal to or less than its length to promote molecular beam flow in gas passing through the hole 6. The hole 6 is fed from a tube (not shown) underneath the plate 4, the tube having the same internal diameter as the hole 6 to assist in promoting molecular beam flow. The cathode 2 has an electron screen 9 having an aperture 10, and the electrode 5 forms a channel 8 and is secured to the plate 4 by bolts 11.

In use, the plate 4 and the electrode 5 are mounted on insulators in the normal manner and are maintained at a potential of 2000 volts positive with respect to the ion target of the spectrometer. The cathode 2 and the screen 9 are maintained at a potential of volts with respect to the plate 4 and the anode 3 is maintained at a potential of +25 volts with respect to the plate 4. A heating current is applied to the cathode 2 to provide an ionizing beam of electrons passing from the cathode 2 to the anode 3 along the channel 8 as indicated by arrows 12. The sample being analysed is fed in gaseous form through the tube and the hole 6 to emerge in the channel 8 having molecular beam flow, as indicated by arrows 13. The gas is ionized by the electrons of the ionizing beam, and the ions are collected at the target by known methods.

The diameter of the hole 6 is less than the width of the channel 8 and of the same order as the elfective width of the ionizing beam, typical dimensions being:

Diameter of hole 6:0.04 inch. Width of channel 8:0.125 inch.

The tip of the cathode 2 protrudes through the aperture 10 in the screen 9, so that it is between the screen 9 and the nearer end of the channel 8. The relative distances of these components are adjusted so that the required ionizing electron beam is obtained with an efficiency exceeding by many times that of conventional designs. The focussing of the electron beam is assisted by a magnetic field as in conventional practice.

The electrode 5 is designed to provide a uniform potential along the path of the ionizing beam where it traverses the molecular beam flow of gas, this being necessary for good resolution.

The skeletal form of the electrode 5, together with concentration achieved using molecular beam flow, limits the amount of adherence by traces of the sample, and thus the memory effect is reduced, a reduction factor of six being typical over the conventional type of ion source.

Ion sources according to the invention have been shown to have a life many times longer than conventional ion sources.

We claim:

1. A mass spectrometer ion source suitable for use with gaseous samples such as uranium hexafluoride, comprising means for providing an ionizing electron beam, means for providing molecular beam flow of the sample to be ionized across said ionizing beam, and an electrode for maintaining a uniform electric field along the path of the ionizing electron beam, said electrode having a base and two similar and parallel upstanding walls which provide a path for said ionizing electron beam, each of said walls having defined therein a series of apertures to substantially reduce the surface area of the wall whereby the electrode has a small surface area while providing for a uniform electric field along the path of the ionizing electric beam Where it traverses the molecular flow beam.

2. A mass spectrometer ion source as claimed in claim 1, wherein said electrode base has a hole defined therein, the hole having a diameter equal to or less than its length to promote molecular fiow of the beam of the sample to be ionized passing through the hole.

3. A mass spectrometer ion source suitable for use with gaseous samples such as uranium hexafluoride, comprising a plate, an electrode mounted on said plate, the

4 electrode having a base secured to the plate and two similar and parallel upstanding walls integral with the base, said base and Walls defining a channel therebetween, said walls each having defined therein a series of apertures to substantially reduce the surface area of the wall, said plate and base having defined therein a hole communicating with said channel, the hole having a diameter equal to or less than its length, a cathode insulatingly mounted on said plate and positioned at one end of said channel and an anode insulatingly mounted on said plate and positioned at the other end of said channel.

No references cited. 

